A number of presently available adenosine analogs possess antitumor activity. Their usefulness has been limited by their rapid metabolism by adenosine deaminase. 2'-Deoxycoformycin, a powerful inhibitor of adenosine deaminase, has been able to potentiate the antitumor activity of several adenosine analogs in vitro and in vivo. The purpose of this work was to study the pharmacokinetics of 2'-deoxycoformycin in monkeys with special attention to the distribution of the drug across the blood-brain barrier and to study the pharmacokinetics and toxicity of 2'-deoxycoformycin in man in conjunction with Phase I clinical trials presently under way.